Lipo Batteries
Introduction
LiPo, or Lithium-Ion Polymer Batteries, are among the great technical advances of the last 20 years. A powerful energy source used in mobile devices, radio controlled aircraft and vehicles, electric vehicles, and robotics.
These batteries have a very high “specific energy, or mass to energy ratio. They pack more power for their weight than any other type of battery, including other Lithium-based power cells.
They are expensive but considering the amount of power stored plus weight saved, Lipos are very efficient.
However, LiPo batteries require more tender loving care than regular batteries. They have very strict rules for charging, discharging, storage, and disposal. By not obeying these rules it can result in poor performance, reduced battery life, or even a fire or explosion.
When used properly though, LiPo batteries are safe and allow you to build projects you couldn’t power up otherwise.
Let’s learn more about LiPo batteries.
LiPo Batteries
You have probably seen a number of batteries that use “Lithium” technology. So how do LiPo batteries fit in? Learning a bit about how they were developed can help answer that question.
LiPo History
Rechargeable Lithium cells and batteries, which are a group of cells, work by using the reversible storage of ions. Developed in the late1970s at Oxford University. The first Lithium-Ion cell created in 1980.
These cells utilize a liquid electrolyte to hold the Lithium ions. Sony produced the first commercial Lithium-Ion battery in 1991 for use in camcorders.
LiPo batteries, released in 1999 by Ericsson for use in mobile phones, replaced the liquid electrolyte with a gel polymer.
Nowadays, LiPo batteries are popular for radio-controlled aircraft, quadcopters, and ground vehicles. They have an excellent power to mass ratio, which allows for long operating time and minimal weight, making them ideal for aircraft.
How LiPo Batteries Work
A single Lipo battery cell comprises a container that contains a polymer gel electrolyte. Each end of the container features a current collector, with one end being the anode and the other being the cathode. There is a separator in the middle of the cell, dividing it into two sections.
Understanding LiPo Specifications
In order to choose the appropriate LiPo battery for your application, understanding its specifications is essential. Terminology may be familiar if you have used other batteries, but some terms may be perplexing.
Let’s end that confusion, shall we?
Voltage and Cell Count
A LiPo battery, like any battery, consists of one or more cells wired in series. It is also possible to have cells in parallel, as we will see later.
The voltage indicated on a LiPo battery is the total value of the nominal voltage of each cell. Nominal voltage is the average voltage of the cell and is 3.7 volts. So, a two cell LiPo battery will have a nominal voltage of 7.4 volts, 3 cell battery 11.1 volts.
A fully charged LiPo cell shows 4.2 volts. Therefore, the 2 cell battery will read 8.4 volts and the 3 cell LiPo battery 12.6 volts.
The minimum safe voltage per cell is 3 volts. Most users never let their LiPo batteries get down to that level.
- A 2S LiPo battery has two cells.
- A 3S LiPo battery has three cells.
- A 6S LiPo battery has six cells.
Battery Capacity
The capacity of a LiPo battery is measured in milliamp hours, or mAh. This is the current a fully charged battery is expected to continuously deliver in one hour before its cell voltage reaches the minimum acceptable level, 3 volts per cell.
So, a LiPo battery rated at 5000 mAh could deliver 5 amperes continuously for one hour before it needs recharging.
But the same battery can actually deliver more current, just for a shorter period of time. If 50 amperes is drawn continuously, the battery would only last a tenth of an hour, which is six minutes.
You must think this means the battery could deliver 300 amps for only one minute, but that may not necessarily be true. This is where the “C” rating comes into play.
C Rating
The “C” rating is the number the rated current can be multiplied by before the battery is in danger of thermal runaway or catching fire. The “C” rating is multiplied by the specified capacity to determine the maximum current you can safely draw.
Some examples:
- 5000 mAh LiPo battery with a 50C rating – You can draw a maximum of 250 amps from this battery. At that rate, it will only last for 1 minute and 12 seconds.
- 2500 mAh LiPo battery with a 30C rating – You can draw a maximum of 75 amps from this battery. But you had better do it quickly, as it will only last for 48 seconds!
Normally you never draw the maximum C rating, it is really there to handle peaks. So, a device consuming 4 amps, occasionally requiring 20, (such as a motor starting or stalled) a 4000 mAh battery with a “C” rating of 5 or higher will work.
Another use for the “C” specification is when charging the battery. Some LiPo batteries can be quick-charged, and the maximum current recommended is also defined with the “C” value. A battery rated at 3000 mAh and a maximum of 3C can be quick charged at 9000 mA, (9 amps). However, it’s generally best to charge at a rate of 1C, 1.5C maximum. This will be discussed later in the charging section.
Internal Resistance
Another important measurement when working with LiPo batteries is the Internal Resistance, or IR, of the cells within the battery.
It is rarely specified on the label, although can usually be found on spec sheet (if you can obtain one). This is because the value changes over time.
The internal resistance is measured in milliohms, and the higher the value is, the less efficient the cell is. Excess resistance in a cell will be dissipated as heat when current flows through the cell.
This resistance will rise as the cell ages. Additionally, the operating temperature can affect this reading.
You require special test equipment to measure internal resistance, you can’t use an ohmmeter or multimetre. Many advanced chargers have a function to measure total resistance. This is the combined internal resistance of all the cells in the battery.
It’s good practice to measure internal resistance (if you have the equipment) when the battery is new. You should also check it periodically. If it rises suddenly, it could be an indication that the battery is approaching the end of its useful life.
Balancing Output
You’ll find two sets of leads emerging from your LiPo battery.
The output leads are the thicker wires, one red, one black, powering the target load.
Another set of wires lead to a smaller sized connector. The number of wires depends upon the number of cells your battery has. These are the balance or monitor outputs.
The balance wires contain a lead running to the output of each of the cells in your battery. They are for three purposes:
- Monitoring individual cell voltage.
- Charging cells independently.
- Balancing the voltage levels in the cells, so that they all match.
- Balancing is one of the more important tasks for LiPo batteries healthy. I will discuss this later.
LiPo Connectors
The main power output of a LiPo battery is via two low gauge wires and terminated with various types of connectors.
Deans Connectors – Probably the most popular LiPo connector used in model aircraft, quadcopters, and RC.
XT-60 Connectors – Becoming more popular, XT-90 and XT-30 connectors also available and easier to solder than Deans.
EC3 Connectors – Made for Horizon Hobby, popular for RC aircraft.
Traxxas – RC car and truck enthusiasts find it highly popular and extremely easy to assemble.
Anderson Power Poles – An old style connector originally designed for HAM radio, although now rarely used due to its large size.
Tamiya Connectors –These connectors are essentially Molex connectors and not commonly used anymore because of their poor performance.
You can also buy adapters to convert one type of connector to another. However, it’s best to choose a battery that matches the one on your device, if at all possible.
LiPo Battery Accessories
There are several accessories that you can purchase to make using and maintaining LiPo batteries a lot easier. They are available on Amazon or at any hobby shop.
Electronic Fast-Acting Fuse
One very handy accessory to have in your kit is an electronic fast acting fuse. It should be temporarily inserted at the output of your LiPo, between the battery and the load you are powering.
It is crucial that you utilize this device to ensure that your load does not create a short circuit with the LiPo battery, which could cause a fatal LiPo fire. You only need to use this fuse during testing, after which you can connect your battery directly to the load.
LiPo Monitors
These affordable devices are primarily used for testing, but they can also be integrated permanently into your device.
The monitors have an LED numeric display, and they connect to the balance or monitoring leads from your LiPo. They cycle through the cell voltages continuously.
Another feature they have is an alarm, a loud buzzer that can be set to trigger when any of the cell voltages drop below a preset threshold. This is a very useful feature, as it will let you know that it’s time to remove the LiPo and recharge it.
Capacity Controllers
It is possible to monitor the output voltages of each cell using a Capacity Monitor. It is more accurate than the LiPo Monitors. It will balance the cells as well.
This is a useful tool for monitoring battery health in the field.
Splitters, Joiners, and Taps
Making your own wiring adapters using silicone insulated wires, heat shrink tubing, and connectors can be a bit complex. It may not have much economic benefit since premade ones are quite inexpensive and professionally made with proper insulation. Therefore, it’s often simpler to purchase premade ones.
For instance, there was a tap that had some higher gauge wires connected to a Dupont connector. You could use this tap to connect to the output of the LiPo. This would allow you to drive a low current device or simply monitor the battery output.
LiPo Charging
In order to charge your LiPo batteries, it’s essential to use a charger specifically designed for them. This is due to the strict charging requirements these batteries have. If used, multi use charger must have the capability to be set to LiPo or Lithium Polymer.
LiPo Charging Basics
To charge a LiPo battery, you should bring the cell voltages up to 4.2 volts. You must ensure not to exceed this voltage.
Make sure to set the number of cells (the “S” rating) on the charger before you start. Many chargers will detect this automatically, but even so, it is always best to confirm that the detection is correct.
There are two methods to charge the battery:
- Charging the whole battery, through the output cable.
- Charging the cells individually, using the balance cable.
Using the balance cable is always better, even if it takes longer to charge the battery. Many advanced chargers use a combination of both methods, which is fine as they monitor the battery during charging. charging.
The safest charging current is 1C, the rated capacity of the LiPo battery. I do not recommend exceeding this, even if the battery’s rating is for a higher charging current. You can use 1.5C if you are in a hurry but check the LiPo’s temperature during charging and reduce the current if the battery gets too hot.
If your battery is new, consider “breaking it in” by charging and discharging it five to eight times before use. This can increase the longevity of the battery.
LiPo Charging Safety
When charging a LiPo battery there are several rules to follow. These rules are important to ensure the safe and proper functioning of the battery.
The number one rule when charging a LiPo battery is to never leave it unattended, even for a second. If you need to leave the charging area, stop the charge.
Most LiPo fires occur during charging, and the most lethal ones typically result from unattended batteries.
There are some extra safety rules in the list below:
Always
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Always check the connections before starting the charger.
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Charge outside, or in a well ventilated area.
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Always have a fire extinguisher nearby when charging.
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Always charge a LiPo on a non-flammable surface.
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Always have a fire extinguisher handy.
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Keep children and pets away from the area you are using to charge the LiPo.
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Keep away from flammable liquids or objects when charging.
Never
- Leave a charging LiPo battery unattended (I know I already said this, but it is so important that it is worth repeating.
- Charge the LiPo inside the device it is powering. Always remove it and place it in a safe area.
- Short circuit the output of a LiPo at any time, but especially during charging.
- Charge a damaged or ballooned LiPo.
When charging your LiPo it is a good idea to have it inside a battery bag or other protective enclosure. And check the battery periodically for excess heat, it’s not uncommon for it to get a bit warm, but if it gets too hot to hold, then stop charging.
Also, observe the battery while charging and look for signs of ballooning. If you observe this, then immediately halt the charging process.
Never charge to a level above 4.2 volts. It’s strongly recommended to balance the cells at the end of the charging process, many chargers will do this automatically.
LiPo Discharging & Balancing
The whole purpose of having a LiPo is to power something which discharges the battery. Unlike other types of batteries, it’s not a good idea just to run a LiPo until it dies.
Monitoring During Discharge
As with petty well every aspect of using LiPo batteries, it’s a good idea to monitor the battery while discharging.
In the discussion about LiPo accessories, I showed you an inexpensive LiPo monitor, the one with an LED display. These are very useful to make sure that your battery is healthy, and is not excessively unbalanced. They will also sound an alarm when the battery approaches a critical low cell voltage.
The Capacity Controller can also be used to periodically test the LiPo cells. One of it’s modes is to display the total voltage difference between the highest and lowest cells. It also displays which cells have this difference, although on a 2-cell (2S) LiPo that would be pretty obvious!
Balancing a LiPo Battery
Another great use for the Capacity Controller is to balance the LiPo. It accomplishes this by determining which cell has the lowest voltage. Once determined it applies a resistive load across the other cell(s). The load will drain the cells until they match the lowest-voltage one.
You can also purchase a separate device that is exclusively made to do this. It operates similarly to the capacity controller.
If you are near your charger, check to see if it has a balancing function. If it has you can use that.
LiPo Storage
LiPo batteries have specific storage requirements. Keeping them in your house or garage is inadvisable due to the risk of fire. In the “real world,” this isn’t always possible.
Let’s see how we can safely store and transport our LiPo batteries.
Storage Voltage
You should never store a fully charged LiPo battery. Bring down to “storage voltage”, which is 3.8 to 3.85 volts per cell.
There are a few ways you can achieve this.
Charge the cells to 3.8 volts it they are below it.
Discharge the cells to 3.8 volts if they are above it.
The best LiPo chargers have a Storage function, which will set the cells to 3.8 volts. This can take a long time if they are nearly fully charged and require discharging.
Batteries can be installed in a device to run them down to or just below storage voltage. Charge them to storage voltage in the charger.
A resistive load can be used to drain the batteries, a 20 – 40ohm power resistor is a good choice. The resistor can get warm, not hot, and the battery should remain cool. Depending upon how charged up they are, it may take some time to complete the drain down to 3.8 volts.
If they do need to be brought to storage voltage, observe all of the safety advice from the charging section. Remember, never leave them unattended.
LiPo Battery Bags
If you search for LiPo storage, there is a good chance that you’ll come up with a number of “LiPo Bags”.
The bags claim to be explosion and fire-resistant which seems like the perfect solution to store your batteries safely. However, in reality, they don’t live up to their advertised claims.
These bags are made of metallic woven cloth, available in various sizes with zipper or Velcro closure.
It is crucial to understand that fireproof bags do not offer complete protection against a LiPo fire, despite their claims. They may succeed in slowing down the spread of fire to some extent, which can buy you some time to use an extinguisher, but they cannot withstand the heat entirely. If left unattended, these bags can break down rapidly, so it is important to take immediate action in case of a LiPo fire.
LiPo bags are useful for specific purposes, but they should not be relied upon as the sole safety measure. They should only be used while charging a LiPo battery. Make sure to always supervise the process and keep a fire extinguisher nearby. Use other solutions for storage. LiPo bags are a handy option for transporting the battery safely from storage to the device it powers.
Using an Ammo Box
Storing LiPo batteries in an ammo box is an affordable and safe option due to its steel construction that provides exceptional fire protection. You can find them in sporting goods stores and also on Amazon.
Before using the ammo box, it is crucial to modify. These boxes come with a rubber seal on the lid, which makes it air and watertight. However, storing LiPo batteries in an airtight container is not recommended. LiPo batteries can release gas, and if the gas cannot escape, it can create dangerous levels of pressure. This pressure can cause an explosion if it ignites, which would be catastrophic.
Always remove the rubber seal from an ammo box before using it as a LiPo storage box The seal is generally very easy to remove. Use a screwdriver to pry it off the box.
Users can improve airflow by drilling small holes in the box but removing the seal should be sufficient.
You can add extra protection by placing the LiPo battery inside a battery bag before placing it in the box.
My Homemade LiPo Storage Box
I built a storage box specifically for my LiPo batteries. To make it, I used a steel toolbox that I bought on Amazon. The crucial aspect is that it is made of steel, which is ideal for fire resistance.
I lined the toolbox with drywall (sometimes called Sheetrock), as it also has excellent fire-resistant properties. I lined the bottom and sides of the box with the drywall and also made a drywall “lid” to cover the batteries.
Once again, I put the batteries inside battery bags for additional protection.
You can easily create a storage box yourself to store or transport multiple batteries.
LiPo Fire Extinguishing
I hope you never experience a LiPo fire. The fire is fast burning and emits toxic chemicals.
Even with necessary precautions, a fire caused by an accident or defective LiPo can still occur. It’s important to be prepared to extinguish the fire quickly.
LiPo Fire Risk
A LiPo battery can experience thermal runaway, a condition in which the heat continues to increase unabated. Several factors can lead to this issue.
- Charging a battery with excessive Internal Resistance in one or more cells.
- Charging a battery that is damaged, defective, or swollen.
- Subjecting a battery to high temperatures beyond its recommended range. Make sure that the temperature stays below 60 degrees Celsius.
- Excessive shock or pressure, such as that experienced in a crash.
Note that the majority of times thermal runaway occurs during charging, so it’s crucial to be prepared for a potential fire.
When a LiPo experiences thermal runaway, it may release toxic gas, which can ignite and cause an explosion. Therefore, charging LiPos in a well-ventilated area, preferably outdoors is important.
Extinguishing a LiPo Fire
The name “Lithium-ion Polymer Battery” can be misleading as it only contains a small amount of lithium. While this small amount can react with water, it is usually burned off within seconds at the start of a fire. As a result, by the time a fire is detected, the lithium would have already been consumed.
Therefore, buying an expensive, hard to use, Class D fire extinguisher is unnecessary, although recommended for a lithium fire.
Instead, a standard Class ABC extinguisher can be used. This is something you should have around your workshop anyway, so you probably already own one. If not, they can be obtained and just about any hardware store.
You can use water during a LiPo fire, but it is best used to hose the area around the burning battery to prevent it from igniting. Often the biggest danger from a LiPo fire is the items surrounding the battery that catch fire. Another great reason to charge your LiPo away from such items!
A great way to quickly extinguish a LiPo fire is with sand. A bucket or bag of sand is pretty easy to obtain, and you can smother the fire with it.
LiPo Disposal
All batteries have a limited lifespan, and LiPo batteries are no exception. If handled correctly, these batteries can last for 350 to 500 charge and discharge cycles, but eventually, they will require replacement.
When they do come to the end of their useful life, they need to be discharged and properly disposed of.
Determining when your LiPo needs Replacement.
If you’ve taken care of your LiPo batteries then they should last a pretty long time, as stated above you should be able to get at least 350 uses out of them.
But if you have abused them by not keeping them balanced or stored properly (at 3.8 volts per cell), or if they have been damaged, then they may not last as long.
If you’re tracking the internal resistance of the cells and notice a sudden increase, then it’s a sign that the LiPo is near the end. If the battery gets warm under normal use, if it is ballooned, or if it fails to hold a charge or stay balanced, then it’s time to replace it.
Discharging a LiPo Battery
Before you can dispose of your LiPo battery, it needs to be completely discharged. Even a “dead” LiPo can hold a lot of energy, and it can catch fire if crushed in a garbage truck or trash compactor.
Many quality LiPo chargers have a “Discharge” or “Destroy” function that will completely drain the battery. The procedure can take several hours, or even more than a day, depending upon how much energy is left in the cells.
Another way of draining a LiPo is to use an automobile taillight or a resistive load, leave it on the LiPo for a day, and then measure the output voltage. If it is more than zero, then put the load back on for a few more hours. Keep going until no electricity can be detected.
One method that many R.C. hobbyists use is to soak the entire cell in a saltwater bath for a day or two. You can also do this after draining the battery with one of the above methods, to ensure that it is completely depleted of energy. Table salt will work fine, you need to saturate the water until it can’t hold any more salt.
Disposing of a LiPo Battery
After discharging the battery completely, you can dispose of it.
In many municipalities, you can simply throw it in with the rest of your trash, as it no longer contains any volatile or dangerous chemicals and has very little lithium. But in some areas, you will need to take it to an authorized disposal facility.
Make sure you check with the proper authorities to determine the proper disposal method for your area. In my community, we have a bi-annual hazardous goods disposal at the city hall, and they take all batteries, including LiPo batteries.
Conclusion
When it comes to using LiPo batteries, nothing is more important than safety. Fortunately, it doesn’t take a lot of effort to observe the proper safety precautions, minimizing the chance of a dangerous fire or outgassing.
LiPo batteries are powerful devices that need to be treated with respect and handled with care. If you observe proper handling, charging, balancing, and storage techniques, these batteries can last a long time and provide a virtually unlimited source of power for your projects.
Deans Connectors – Probably the most popular LiPo connector, used in model aeroplanes and quadcopters as well as RC vehicles.
XT-60 Connectors – Becoming very popular, easier to solder than Deans. There are also larger XT-90 and smaller XT-30 connectors.
EC3 Connectors – Made for Horizon Hobby, popular for RC aircraft.
Traxxas – Very popular with RC Cars and Trucks, very easy to assemble.
Anderson Power Poles – An older style connector, originally designed for HAM radio. Not used too often due to their size.
Tamiya Connectors – Essentially these are Molex connectors, not used much anymore as they have poor performance.
You can also buy adapters to adopt one type of connector to another one. However, it’s best to choose a battery that matches the connector on your device, if at all possible.
Deans Connectors – Probably the most popular LiPo connector, used in model aeroplanes and quadcopters as well as RC vehicles.
XT-60 Connectors – Becoming very popular, easier to solder than Deans. There are also larger XT-90 and smaller XT-30 connectors.
EC3 Connectors – Made for Horizon Hobby, popular for RC aircraft.
Traxxas – Very popular with RC Cars and Trucks, very easy to assemble.
Anderson Power Poles – An older style connector, originally designed for HAM radio. Not used too often due to their size.
Tamiya Connectors – Essentially these are Molex connectors, not used much anymore as they have poor performance.
You can also buy adapters to adopt one type of connector to another one. However, it’s best to choose a battery that matches the connector on your device, if at all possible.
Deans Connectors – Probably the most popular LiPo connector, used in model aeroplanes and quadcopters as well as RC vehicles.
XT-60 Connectors – Becoming very popular, easier to solder than Deans. There are also larger XT-90 and smaller XT-30 connectors.
EC3 Connectors – Made for Horizon Hobby, popular for RC aircraft.
Traxxas – Very popular with RC Cars and Trucks, very easy to assemble.
Anderson Power Poles – An older style connector, originally designed for HAM radio. Not used too often due to their size.
Tamiya Connectors – Essentially these are Molex connectors, not used much anymore as they have poor performance.
You can also buy adapters to adopt one type of connector to another one. However, it’s best to choose a battery that matches the connector on your device, if at all possible.